Relating to radio frequency sputtering

ABSTRACT

A METHOD AND APPARATUS FOR RADIO FREQUENCY SPUTTERING OF AN INSULATING TARGET BY MAKING THE TARGET PART OF THE WALL OF AN EVACUABLE SPUTTERING DEPOSITION CHAMBER AND ATTACHING A RADIO FREQUENCY ELECTRODE TO THE TARGET OUTSIDE THE DEPOSITION CHAMBER IN A FURTHER EVACUABLE REGION. THE FURTHER REGION IS REDUCED IN PRESSURE WHEN A RADIO FREQUENCY VOLTAGE IS APPLIED TO THE ELECTRODE SO AS TO PROHIBIT EARTHY DISCHARGES.

cs. N. JACKSON 3,558,467

RELATING TO RADIO FREQUENCY SPUTTERING Jan. 26," 1971 2 Sheets-Sheet 1Filed June 21, 1968 INVENTOR BY M,

ATTORNEY (5. N. JACKSON RELATING TO RADIO FREQUENCY SPUTTERING Jan. 26,1 971 Filed June 21. 1958 2 Sheets-Sheet I ATTORNEY United States PatentUS. Cl. 204-298 3 Claims ABSTRACT OF THE DISCLOSURE A method andapparatus for radio frequency sputtering of an insulating target bymaking the target part of the wall of an evacuable sputtering depositionchamber and attaching a radio frequency electrode to the target outsidethe deposition chamber in a further evacuable region. The further regionis reduced in pressure when a radio frequency voltage is applied to theelectrode so as to prohibit earthy discharges.

This invention relates to radio frequency sputtering of insulators.

Sputtering is a term to define a vacuum process in which a target isbombarded by energetic ions or atoms so that the target is eroded, theeroded material being sputtered olf the target and transported throughthe environment to be deposited on any suitable available surface. Suchsurfaces can be provided by substrates which are desired to be coated bythe target material. There are many common techniques for controlledsputtering of metal targets, but these fail when the target comprises aninsulator. This is due to the fact that the bombardment of an insulatorwith charged particles gives rise to a build up of charge on theinsulator surface. This charge gives rise to a potential which repelsthe bombarding particles so that sputtering very soon decreasesconsiderably or even stops.

However, the sputtering of insulators has been achieved by the techniqueknown as radio frequency (R.F.) sputtering. Basically this consists ofimmersing a dielectric target which is backed by a metal electrode in aplasma. A suitable R.F. voltage is then applied to the electrode so thatthe front face of the dielectric target takes up an alternating voltagedue to the capacitive coupling with the electrode. Due to the greatermobility of electrons than that of ions in the plasma, the R.F. voltageresults in a net D.C. negative bias being built up on the front surfaceof the target and energetic ions are permitted to bombard it wherebysputtering takes place.

R.F. sputtering techniques fall into two classes, those using a plasmagenerated in a glow discharge between a thermionic cathode and an anode,and those using the R.F. electric field to generate an R.F. glowdischarge. For both these techniques the plasma is generated in a regionwhich has a charateristic pressure typically in the range X to 10 torr.

It is desirable that breakdown discharges are prevented from occurringbetween metal electrodes-Le. between an R.F. electode and earthsincethis reduces or prevents sputtering of the dielectric while sputteringof the electrode takes place. Such breakdown discharges have beenprevented in the past by three methods. These are diagrammaticallyillustrated in FIG. 1, FIGS. 2 and 3 and FIGS. 4 and 5 respectively ofthe accompanying drawings. FIG. 1 shows an R.F. electrode 1 associatedwith a dielectric target 2, the breakdown discharge being prevented byproviding a solid dielectric shield 3 which shields those parts of theelectrode not shielded by the target. This arrangement is not normallyused with a R.F. generated plasma because it does not utilise R.F. powerefficiently.

FIG. 2 shows a R.F. electrode 1, the top surface of which is providedwith the dielectric target (not shown). The electrode is surrounded byan earthed metal shield 4 situated at a distance from the target whichis insufiicient for a breakdown discharge to occur. FIG. 3 shows asimilar arrangement with twin R.F. electrodes 1 and 5, the arrangementsurrounded by a similar earth shield 4. In these arrangements theseparation between the electrode and the shield must not be too smallbecause the capacitance effects increase as the separation decreases.However, if the separation is too large, electrons accelerated acrossthe gap ionise enough atoms of the environmental gas for the undesirablebreakdown to occur. The capacitance effects referred to are undesirableas they lead to some or all of the following:

(a) frequency change;

(b) change in electrode load impedance;

(c) loading of the supply so that it fails to operate or is unstable inoperation.

The maximum gap allowable to decrease these effects is dependent on theenvironment gas and its pressure. Typically a A" gap at a pressure of 510 torr is used for argon.

The third method is shown in FIGS. 4 and 5, which show arrangements fora R.F. self generated plasma and an anode/ cathode generated dischargerespectively. FIG. 4 shows a vacuum chamber 6 having a pumping outlet 7and an end plate 8 which comprises the dielectric target. A metalelectrode 1 is attached to the outer surface of the dielectric plate andis thus located in a region at atmospheric pressure. The other side ofthe R.F. supply 5 is grounded and an earthed plate 9 serves to balancethe self generated plasma. The plate 9 may also be arranged to supportthe work or substrate to be covered by the sputtered dielectricmaterial. FIG. 5 also has a chamber 6, a pumping outlet 7, a dielectrictarget 8 which forms a limiting surface of the chamber and an R.F.electrode 1. In this case the discharge occurs between a cathode 10 andan anode 19. These arrangements are suitable in that the R.F. electrodeis, in each case, at atmosphere pressure whereby breakdown to earth canbe eliminated. However, the are not used for high R.F. powers becauseerosion of the dielectric target 8 weakens it so that it may not be ableto withstand the pressure differential across its surfaces and there isa danger of implosion.

It is an object of the present invention to provide an apparatus whichmay be operated safely and without any substantial risk of breakdowndischarges occurring between the R.F. electrode or electrodes and earth.

According to the present invention a method of radio frequencysputtering comprises the steps of providing a first evacuable chamberwith a limiting wall portion which comprises a target of insulatingmaterial, said evacuable chamber being the sputtering depositionchamber, attaching a target electrode to the surface of the said targetoutside the first evacuable chamber, reducing the pressure within thefirst evacuable chamber to that pressure required for sputtering of thetarget, reducing the pressure outside the said first chamber and in theregion of the electrode to such a value that breakdown discharges fromthe electrode are unlikely during use of the apparatus, and applying aradio frequency voltage to the electrode in such conditions thatsputtering of the target is achieved.

Radio frequency sputtering apparatus for carrying out the above-methodincludes a first evacuable chamber or region, being the sputteringdeposition chamber, a metal target electrode and a target of insulatingmaterial in faceto-face relation, the face of the target remote from theelectrode providing a part of the interior surface of the firstevacuable chamber or region and the electrode being situated in a secondevacuable chamber or region.

When using such an apparatus the evacuable region containing theelectrode should be kept at a pressure which is low enough almostcompletely to prevent any possibility of an earthy discharge oecuringbetween the electrode and any other metal (i.e. earthed) components.

In the case of a twin R.F. electrode system, or any R.F. system withmore than one R.F. electrode, each R.F. electrode will be located in anevacuable chamber other than the sputtering chamber.

The invention will now be described in greater detail, by way ofexample, with reference to FIGS. 6 and 7 of the accompanying drawings inwhich:

FIG. 6 is a diagrammatic representation of sputtering apparatusembodying the invention; and

FIG. 7 is a diagrammatic representation of an RF. electrode arrangementof a modified apparatus embodying the invention.

Referring now to FIG. 6, a vacuum chamber 11 is provided with a pumpingoutlet 12 and contains an inner chamber 13. Chamber 13 comprises a glasstube 14 closed at one end by a metal earthed plate 15 and at the otherend by an insulating target 16. Attached to the outer surface of target16 is an R.F. electrode 17 to which an R.F. voltage is applied by meansof lead 18. As shown in FIG. 6, plate 15 is provided with an aperturewhich is connected as by a conduit 22 to a pumping system 23.Alternatively the chamber 14 may be pumped by a small restrictiveaperture leading to chamber 11. There is also (not shown) an inletthrough plate 15 in the form of a gas jet for introducing gas to anydesired pressure. It will thus be seen that chambers 11 and 14 can bothbe evacuated, but to different pressures. In operation one side of theR.F. supply is grounded whilst the other side is applied via line 18 tothe electrode 17. With a Single R.F. electrode 17 a plasma can begenerated at a pressure in the region of torr. Some difiiculty isexperienced in its generation at lower pressures although once startedit can be sustained at a pressure which is lower by about a factor of10. The combination of evacuation through the aperture (not shown) inplate and introduction of gas through the jet (not shown) permits thechamber 14 to be brought to the desired process pressure in a desiredenvironment, for instance argon. The plasma generated in chamber 14causes sputtering of the target 16 for deposition on a substrate orsubstrates which may conveniently be fixedly or rotatably attached to ormounted on plate 15 by suitable supporting means, indicated at 24.

Independently of chamber 14, chamber 11 is pumped down to a pressurewhich is low enough, say not substantially above 10- torr, to prevent adischarge between electrode 17 and any earthed metallic chambercomponents. The differential pressure across the target 16 is nowminimal compared with that in the arrangements of FIGS. 4 and 5 andthere will be little or no danger of implosion.

Clearly, if more than one R.F. electrode was to be used, each would bearranged within chamber 11 with a capacitive coupling through one ormore dielectrics to the interior of chamber 14.

In small pumping systems, of the order of 12" sputtering chamberdimensions, pressures up to about 5 l0 torr may exist in chamber 11although this will ultimately depend on the geometry of the system. Insmaller systems in which the components spacings are 10 ems. or lesshigher pressures can be tolerated as there will be less likelihood ofdischarges being formed between the R.F. electrode or electrodes andearth components as the electron paths will be too small for ionisationto occur. Pressures of 10- torr or higher may exist Within thesputtering chamber 14 depending upon the sputtering conditions required.

FIG. 7 shows an electrode arrangement which is a combination of lowpressure electrode environment, as required for operation of theinvention, and dielectric shielding. Thus the insulating target 16 hasone face open to a sputtering chamber (not shown) and its other faceattached to a R.F. electrode 17 arranged in tube 20 c0nnected to avacuum system which maintains a sulficiently low pressure to prevent adischarge from occurring.

The surface of the electrode facing the walls of the tube 20 areshrouded by a dielectric shield 21 which ensures breakdown will notoccur between these surfaces and the walls of the tube under typicaloperating conditions. This electrode arrangement may be used for a selfgenerated plasma such as is shown in FIG. 6 or in a system similar tothat illustrated in FIG. 5 in which the glow discharge is provided by ananode/cathode.

From the above it will be seen that use of apparatus as described issuitable for sputtering at widely differing pressures without thenecessity of providing earth shielding of the R.F. electrode orelectrodes. As is well known, sputtering rates can be enhanced and alower pressure self generated plasma using a single R.F. electrode canbe obtained if means are provided for setting up a magnetic field in theregion of the R.F. electrode.

I claim:

1. Radio frequency sputtering apparatus including in combination:

(a) a first evacuable chamber;

(b) a second evacuable chamber;

(c) a metal electrode supported in said first chamber and arranged forconnection to a radio frequency power supply;

(d) an insulator target in surface contact with said metal electrode andcomprising a wall portion separating said first chamber and said secondchamber;

(e) means for maintaining said chambers at different pressures; and

(f) means for supporting a substrate in said second chamber.

2. Apparatus according to claim 1 in which said second chamber issituated within said first chamber.

3. Apparatus according to claim 1 including a dielectric shield coveringan otherwise exposed surface of said electrode.

References Cited UNITED STATES PATENTS 3,486,935 12/1969 Eyrich 2042983,471,396 10/1969 Davidse 204-298 3,391,071 7/1968 Theverer 204298 JOHNH. MACK, Primary Examiner SIDNEY S. KANTER, Assistant Examiner

